In a magnetic storage system utilizing hard disks, the recording density, and therefore the ultimate data storage capacity of a disk, is determined by a number of factors including the thickness of the recording media and the characteristics of the magnetic transducer which interacts with the disk to read and write data. One of the more significant characteristics of the transducer is the operating height (distance) from the disk. Positioning the transducer closer to the disk allows denser recording. While a transducer in continuous direct contact with the disk would theoretically allow the highest density recording, such an approach is undesirable from the practical standpoint since the transducer or the magnetic coating on the surface of the hard disk would rapidly wear to the point of inoperability.
If, on the other hand, a system is designed in which the transducer touches the disk for only short periods of time and then only during start up and stop operations when the disk is rotating relatively slowly, wear is reduced but recording density is sacrificed.
Various slider designs have been devised to permit operation of the slider in ever closer proximity to the surface of the disk. Improvements in slider aerodynamics have led to increased stability and "stiffness" but the necessity for avoiding high speed contact with the surface of the disk requires a safety factor in the flying height. For this reason, the ultimate performance of a given slider design can never be realized.
While some contact with the disk can be tolerated without substantially increasing the possibility of system failure, the performance of existing slider designs is such that any contact induced wear causes a decrease in the fly height which results in more contact with the disk and a rapid further increase in the wear. In other words, the wear induced changes in slider performance operate to change the slider performance in a way to accelerate wear in a fashion which leads to premature failure of the storage system.
The common bi-rail taper-flat air bearing slider configuration used for thin-film or ferrite heads is designed to fly without contacting the disk except at start-up and shut-down, when the disk is operating at reduced speed. If the head design is altered to reduce the fly height to the point where occasional contact is made when the disk is at operating speed, the head will wear at the back end (trailing edge). As a consequence of the wear, the fly height is reduced and the head wears more. This process continues until the head to disk contact reaches the point where destruction of the head and/or the disk surface occurs.
In the common type of slider, the pressure on the back of the rails is normally higher than the pressure further forward since the air is compressed proportionally more at the back of the slider than near the middle of the slider. As the air under the rails is compressed, some loss of pressure occurs due to leakage out from under the sides of the rails, tending to reduce the pressure increase near the back of the rails. If, by reason of wear due to contact with the disk, the rear portion of the rails become flat (parallel) to the disk surface, there is no further pressure increase under the back of the rails. The air under the slider is compressed as it moves to the rear of the slider since the rails are closer to the disk at the rear of the slider. However, when the rear position of the rails becomes worn and lies parallel to the surface of the disk, there is no further compression and no resulting pressure increase. In fact, due to the air which leaks out from the sides of the rail, there is a drop in pressure near the back of the slider. The drop in pressure at the rear has two adverse effects. First, the reduced pressure at the rear of the slider reduces the support for the slider in the region near the back of the slider causing it to fly closer to the disk and accelerate wear. Secondly, there is no attendant decrease in pressure at the front of the slider so there is a tendency for the slider to pitch upward, which, after some wear has occurred, also causes the back of the slider to move closer to the disk surface. Since the two dominant effects of wear both tend to move the rear of the slider closer to the disk and this in turn causes even more wear, the process accelerates until destruction.